In some electric motor systems, alternating current (AC) power comprising three input voltages, each 120 degrees out of phase with the next, is converted by power rectifiers to direct current (DC) power for driving electric motors. Under certain circumstances, such as an electrical short, a phase can be lost. When a phase is lost, the motor can still operate by drawing more current from the remaining phases, but this can overheat the motor's components, especially when operating at full load, and can even overload the transformer. In that light, it is desirable to detect and address the loss of a phase. Prior attempts to detect the loss of a phase include examining the AC input voltages before the power rectifier, with a drop in line voltage potentially indicating the loss of a phase, but this requires multiple isolated or differential circuits. It is also known to examine voltage peaks after the power rectifier, with the absence of an expected peak indicating the loss of a phase, but this requires using a timer to measure the frequency of the voltage peaks.
Power Factor Correction (PFC) circuits are sometimes used to manage power flowing to motors. PFC makes a motor-driven appliance appear to be purely resistive by eliminating any phase difference between the voltage and the current from the power supply, and thereby reduces energy consumption by minimizing inefficient and costly reactive loads, maximizes the available power that can be drawn from the power supply, and minimizes any transient/harmonic effects that can feed back into the electrical system and disrupt the power source to other appliances. Without PFC, an imbalance between input phases, resulting from a decrease in or loss of a phase, results in the current in the reduced/lost phase dropping very low or to zero such that the current drawn by the motor is pulled through the remaining phases. Some systems sense the bus voltage and when input voltage is low and load is high they operate the PFC in a current-limited mode. However, running in this mode for an extended period of time may introduce undue stress on components of the system and adversely affect the reliability and longevity of the system. Importantly, this solution does not sense the loss of a phase in three-phase systems.
This background discussion is intended to provide information related to the present invention which is not necessarily prior art.